Method of manufacturing powder particles

ABSTRACT

The invention relates to a method of manufacturing a dry powder particle, preferably using electro-hydrodynamic spraying, wherein two oppositely charged aerosol streams are contacted. The invention allows for the manufacture of powders having various, controllable compositions and shapes. In particular the method according to the invention may be used to perform physical and chemical reactions and allows for the manufacture of powders not previously obtainable. In addition, the invention relates to an electrode and an apparatus for applying the method according to the invention.

FIELD OF THE INVENTION

This invention relates to a method of manufacturing a dry powderparticle by generating a charged aerosol stream comprised of chargedinitially liquid particles and converting said stream into powderparticles.

BACKGROUND OF THE INVENTION

Such a method is well known in the state of the art. By passing aliquid, comprising a solute dissolved in a solvent, through a narroworifice using high voltage, a charged aerosol stream is generatedcomprised of charged liquid particles. Evaporation of the solventresults in a fine powder with a relatively narrow size distribution.This, process, known as electrohydrodynamic spraying, is for examplesuitable for the manufacture of polymeric powders used for electrostaticspraying during powder coating.

The charged powders manufactured according to the state of the art tendto form agglomerates, making them less suitable in the manufacture ofhigh quality finished surfaces. The charged powders also tend to clog uppipelines, which interferes with both production of the powder and usethereof. Thus, the method according to the invention provides a powderhaving an improved industrial applicability while in addition a higherpowder yield is obtained.

In the field of fuel injection it is known to study the phenomenon ofmixing two aerosol fuel streams by contacting two similarly chargedaerosol fuel streams. (Dunn P. F. et al. the mixing ofelectrically-charged droplets between and within electrohydrodynamicfine sprays, J. Aerosol Sci. vol. 25, 0.6, pp. 1213-1227, 1994).

It is also known to manufacture a homogeneous mixture by contactingcharged granules with an oppositely charged powder (Thesis of P.Vercoulen; Electrostatic processing of particles. Technical Universityof Delft, the Netherlands). To this end a powder, comprising particlesin the micron range, is sprayed and subsequently charged, for exampleusing a corona discharge device, before being contacted with oppositelycharged solid granules having diameters in the order of 2 mm. Theparticulate starting materials were manufactured using conventionaltechniques, i.e. grinding.

SUMMARY OF THE INVENTION

The objects of the present invention are to expand the applicationpossibilities of said method, and in particular to provide a methodallowing for the manufacture of powders which could not be produced sofar, and to improve the quality of powders produced using said method.

To this end the method according to the invention is characterized inthat the charged aerosol stream comprised of charged particles iscontacted with a second aerosol stream comprising oppositely chargedparticles resulting into a combined aerosol stream to form the powderparticle.

Electrohydrodynamic spraying allows for the generation of chargedaerosol streams having well defined particle size distributions, whereinthe liquid particles are charged at the instant the liquid particle isformed, i.e. the droplets do not have to be charged afterwards in aseparate charging step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Thus the present invention provides a method for the manufacture ofpowder particles, which together constitute a powder, which powder iscomprised of less charged or substantially neutral particles. The methodallows for, for example, the use of aerosol streams of differentcomposition enabling physical and chemical reactions to occur, and forthe coating of particles.

According to a preferred embodiment of the invention the amount ofcharge of the first aerosol stream and the amount of charge of thesecond aerosol stream are controlled to yield a substantially neutralpowder.

Thus a higher yield of powder can be obtained than previouslyachievable.

According to a preferred embodiment the particles of at least the firstaerosol stream comprising charged particles are generated byelectrohydrodynamic spraying.

According to a preferred embodiment the first aerosol stream comprisesparticles comprising a solvent and a solute, said solvent beingevaporated to yield a substantially dry powder particle.

This allows for the manufacture of a powder particles comprising two ormore attached subunits. This method is characterized in that a part ofthe solvent is evaporated from the first aerosol stream to form a firstaerosol stream being composed of solid, sticky charged particles whichis contacted with the second charged aerosol stream comprising solidcharged particles resulting in the combined aerosol stream which isconverted into an essentially dry powder. Thus it is possible to makeaerosol particles with a specific shape, the subunits having the same ora different composition.

A particularly preferred embodiment of the method according to theinvention is characterized in that, when contacted, both the first andthe second aerosol stream comprise liquid charged particles.

This embodiment allows for a multitude of physical and chemicalreactions to occur.

Advantageously the method according to the invention comprises, aftercontacting the first aerosol stream with the second aerosol stream,separation of particles with the desired composition according to theircharge to mass ratio.

As the amount of charge is proportional to the mass of the particle (or,in case of a particle comprising an evaporating solvent, to the mass ofthe solute), the composition of the particle is reflected in the chargeto mass ratio of the particle formed.

Yet another preferred method of manufacturing a powder according to theinvention is characterized in that at least the first aerosol stream,being comprised of larger particles and satellite particles, issubjected to a particle separation step providing a substantiallymonodisperse aerosol stream whereafter the substantially monodisperseaerosol stream is contacted with the oppositely charged aerosol stream.

This embodiment, which is especially useful with electrohydrodynamicspraying, provides for a powder to be produced having a even narrowerparticle size distribution. Accordingly it is preferred that both thefirst and the second aerosol stream are subjected to the particleseparation step.

According to an advantageous embodiment, the separation step isconducted using a grounded electrode. The grounded electrode, placednear or in that part of the aerosol stream where the particles to beremoved pass and more distantly spaced from the desired particles,serves as a simple and effective means to attract and remove theunwanted particles. Thus an aerosol stream enriched in the desiredparticles is obtained and subsequently brought into contact with theoppositely charged aerosol stream.

The invention also relates to powder comprising ceramic precursor powderparticles, polymer-comprising powder particles or powder particlescomprising a pharmaceutical compound, for example coated powderparticles, as well as a pharmaceutical composition comprising such apharmaceutical compound-comprising powder together with apharmaceutically acceptable carrier or diluent.

Moreover the invention relates to a method of manufacturing a ceramicproduct characterized in that a ceramic precursor powder according tothe invention is sintered.

The method allows for the manufacture of very fine ceramic precursorpowders without grinding as well as powders with compositions whichuntil now could not be obtained or only with great difficulty. If theabove mentioned separation step is performed, providing two aerosolstreams comprised of satellite droplets, highly desired ceramicprecursor nanoparticles can be obtained, suitable for defect freeceramic products.

Finally, the invention relates to an apparatus for working the methodaccording to the invention, comprising an electrode having an inlet andan outlet, a channel connecting the inlet and the outlet, the outletbeing an orifice in the centre of an electrically conducting areaopening into a chamber, wherein the electrically conducting area isdefined by a barrier, said barrier preventing the flow of liquid beyondthe electrically conducting area and the chamber having a counterelectrode as well as an exhaust opening for a product formed byelectro-hydrodynamic spraying.

The apparatus is characterized in that the chamber is a mixing chamberand the apparatus is provided with a second electrode having an orificeopening into the chamber.

The invention will be hereinafter explained in more detail withreference to the drawing, in which the only FIGURE represents aschematic cross sectional view of an apparatus for working the methodaccording to the invention.

The present invention provides a method for contacting a first chargedaerosol stream comprised of charged particles with a second aerosolstream comprised of oppositely charged particles in a mixing zoneresulting into a combined aerosol stream which is subsequently convertedinto a powder. Particles making up an aerosol stream have a diameter of100 μm or less—usually in the micron range.

Advantageously the first charged aerosol stream is generated usingelectrohydrodynamic spraying, which method is well known in the art. Aliquid A is passed via a channel 1 through a narrow orifice 2, forexample with a diameter of 0.2 mm, of a nozzle 3. A high voltage,typically 5-30 kV is supplied over the nozzle 3 and an annular groundedcounter electrode 4. The high voltage accelerates the liquid A throughchannel 1 and towards the annular counter electrode 4. At the orifice 1a liquid jet B emerges which breaks up into charged droplets forming aconical stream of droplets having narrow size, charge density andvelocity distribution. Because of their initial speed the droplets donot encounter the counter electrode 4. During the breaking up processsmaller droplets, known as satellite droplets, are created as well,resulting in a bi-modal particle size distribution. These satellitedroplets, being very small, end up in the outer periphery of the conicalaerosol stream.

If desired it is possible to eliminate one of both types of droplets bysuitably placing a grounded electrode 5 near its path, for example inposition C for the removal of the satellite droplets or in position Dfor the removal of the larger droplets.

The charged aerosol stream comprised of charged droplets is contactedwith an oppositely charged second aerosol stream in a mixing zone 6.This second aerosol stream may be comprised of liquid particles, i.e.droplets, or solid particles.

Due to the opposite charge of the particles of each aerosol stream, theparticles of the different aerosol streams are attracted to each otherand combine, forming less charged and even neutral particles.

Advantageously the amount of charge of the first aerosol stream and theamount of charge of the second aerosol stream are controlled to yield aneutral powder. This can be achieved in several ways, two of which arediscussed. Firstly, the amount of charge per second of the first aerosolstream should be equal to the amount of charge per second of the secondaerosol stream. Secondly, if one of the streams consists of very manyfine particles in comparison to the particles of the other stream, manyfine particles will be needed to neutralize a large particle, allowingaccurate neutralization of the large particle. Thus it may be possibleto increase the yield of neutral particles. An aerosol stream comprisingvery small particles may be obtained exploiting the Rayleigh break-upphenomenon, which occurs when the charge density in a particle becomestoo high, due to evaporation of a solvent. The very fine particles maybe used in excess, the surplus being wasted. To waste the surplus, acharged electrode may be used and advantageously the very fine particlesconsist of a volatile solvent only.

Mixing of the aerosol streams can be enhanced by supplying a gas G,usually an inert gas, to the mixing zone 6 where the first and thesecond aerosol stream are contacted. The gas flow also provides aconvenient way to carry off the powder formed. The gas G may be suppliedthrough inlets 9 and prevents space charge accumulation in front of thenozzles 3, which would result in an increase in electrical fieldintensity and unstable aerosol streams.

The method according to the invention results in a powder comprised ofless charged or substantially neutral particles. Charged productparticles may be removed using a grounded electrode yielding a productstream of substantially neutral particles, or classified according totheir charge to mass ratio using charged electrodes 10 a, 10 b. Such aseparation may be very valuable, for example to select particles with adesired composition. The manufacture of powders of various compositionwill be discussed below.

As stated earlier, the second aerosol stream may be comprised of liquidparticles, i.e. droplets, or solid particles. If the second aerosolstream is comprised of solid particles these may have been chargedtribologically or by using for example a corona discharge device.However, preferably the second aerosol stream is generated (both forsolid and liquid particles) using electrohydrodynamic spraying as well,as shown in the figure, wherein parts indicated with reference numeralswith an apostrophe correspond to those mentioned above having the samenumber. Using a solute-containing liquid, evaporation of the solventbefore contacting the stream results in charged solid particles quicklyand efficiently.

If the first aerosol stream is composed of liquid particles whilecontacting the second aerosol stream, it is possible to manufacturecoated particles. Coated particles are for example of interest in themanufacture of pharmaceutical compositions, for example for the quick,delayed or sustained release of a pharmaceutically active compound. Whenit is desired to coat a liquid particle, it may be necessary toincorporate a surfactant.

Contacting the first charged aerosol stream with a second chargedaerosol stream of liquid particles opens a whole range of possibilitiesto manufacture particles with various composition.

According to a preferred embodiment the liquid of the first aerosolstream comprises a first agent and the liquid of the second aerosolstream comprises a second agent. Thus it is possible to perform manyphysical and chemical reactions in very tiny droplets.

For example, if the liquids of the first and the second aerosol streamare miscible, homogeneous physical or chemical processes may occur ineach newly formed droplet. If the liquids are immiscible heterogeneousprocesses may occur. Typical physical processes include precipitationand crystallization processes. Thus the method according to theinvention may, for example, result in particles each comprising anamorphous mixture of compounds which were originally present in each ofthe aerosol streams, or a crystal. In this way it is for examplepossible to manufacture ceramic precursor powders. Amongst the ceramicprecursor powders those for the manufacture of high temperaturesuperconductors can be mentioned.

A potentially useful embodiment comprises contacting a water miscibleorganic solvent comprising a not or sparingly water soluble polymer withan oppositely charged aqueous aerosol stream, the water causing thepolymer to precipitate. Another simple way to obtain a precipitate is byusing an alkaline and acid liquid for the respective aerosol streams, ifthe solubility of the solute is pH dependent.

A preferred embodiment of the method according to the invention ischaracterized in that the liquid of the first aerosol stream comprises afirst reagent and the liquid of the second aerosol stream comprises asecond reagent and the reaction comprises a chemical reaction. In thepresent application the term “reagent” includes chemical or biologicalcatalysts, for example an enzyme.

Due to chemical reactions the product particle may contain a newcompound. The chemical reaction may be virtually any chemical reaction,for example a polymerisation reaction.

In case of a heterogeneous reaction, the invention allows for thepreparation of compounds which can not be obtained efficiently by mixingthe liquids in bulk, for example due to the formation of a solidreaction product preventing the remaining reagents from reacting.

The invention also relates to an electrode, suitable for use with themethod according to the invention employing electrohydrodynamicspraying, having an inlet and an outlet, a channel connecting the inletand the outlet, the outlet being an orifice in the centre of anelectrically conducting area 7, wherein the electrically conducting area7 is defined by a barrier, said barrier preventing the flow of liquidoutside the electrically conducting area 7.

According to the invention the electrode is characterized in that theelectrically conducting area 7 is surrounded by a second area 8 ofelectrically conducting material, extending outwardly over at least 1 mmin radial direction from said barrier, preferably over a distance of atleast half of the radius of the first area.

Thus a more homogenous electric field is generated, with a reducedradial component of the electrical field, near the nozzle 3. This isvery important to avoid electrical discharges which would result indisturbed mixing of the first and second aerosol stream. The use of saidelectrode improves the size, charge density and velocity distribution ofan aerosol stream generated using said electrode. In addition theelectrode allows for the use of liquids with higher conductivities thanwith electrodes according to the state of the art.

The barrier may be, as shown in the figure, a steep recess, the wallthereof being for example at an angle of 90° with the first area.Preferably the first and second area lie in the same plane.Advantageously they are electrically connected.

It should be clear from the above, that the method according to theinvention can be used to manufacture, a powder consisting of lesscharged or substantially neutral particles wherein each particlecontains each of the compounds used, or a powder consisting of particlescomprising a compound not present in the starting materials.

The present invention allows for the manufacture of powders which werepreviously only obtainable using emulsion techniques, which require theevaporation/removal of the solvent, making the techniques cumbersome andexpensive.

The invention may also be used to control the size, structure and shapeof the particles that make up the powder. By controlling the rate ofevaporation of a solvent used, the person skilled in the art can achievepowders of, for example, porous, hollow or massive particles. Porous oramorphous particles may find application as catalysts. For coatedparticles, the thickness of coatings can be controlled by suitablechoice of the solute concentration and droplet size. The rate ofevaporation can be controlled by a suitable choice of solvent, heating(micro-wave, gas supplied at a particular temperature), suitable choiceof the aerosol streams etc., as is well known by the person skilled inthe art.

It will be appreciated that the present invention, as claimed in theappended claims, can be worked in several ways, as will be obvious forthe person skilled in the art. For example, if mention is made of aliquid, the liquid may also comprise a molten solid. It is possible toimprove the size distribution of an aerosol stream by, for example,superposing a high frequency alternating voltage on top of the constantvoltage.

Rayleigh-break up may be used to generate droplets even smaller thansatellite droplets and thus allow for the manufacture of extremely finepowders. Preferably a separation step is carried out before the aerosolstream generated is contacted with the second aerosol stream, asdescribed above.

Apart from those already mentioned, the powders according to theinvention will have many other uses, for example for calibrationpurposes and as seed material.

For scaling up the method according to the invention, an array ofnozzles can be used, for example rows of nozzles with alternating a rowfor positively charged aerosol streams and a row for negatively chargedaerosol streams. Advantageously, the nozzles are supplied with liquidfrom one supply, or in case of different liquids from two supplies only,and likewise the voltage can be supplied using one power supply for eachpolarity only.

The apparatus according to the invention may be provided with a movingcounter electrode and/or electrode for the removal of charged particles,for example realised as a conveyor belt. Any particles adhering to theelectrode are removed outside the mixing zone, for example outside thechamber, and are—depending on the type of powder formed—reused ordisposed of.

What is claimed is:
 1. Method of manufacturing powder particlescomprising: (a) generating a first aerosol stream of charged liquidparticles, (b) generating a second aerosol stream of charged particles,(c) contacting at least one particle in the first aerosol stream with atleast one particle in the second aerosol stream to form powder particlesdifferent from the at least one particle in the first aerosol stream andthe at least one particle in the second aerosol strewn; and (d)separating a powder particle from the powder particles according tocharge to mass ratio of the powder particles by subjecting the powderparticles to an electric field.
 2. Method of manufacturing powderparticles according to claim 1, wherein the first aerosol stream iscomprised of liquid particles comprising a solvent and a solute. 3.Method of manufacturing powder particles according to claim 2,comprising evaporating the solvent after contacting the at least oneparticle in the first aerosol stream with at least one particle in thesecond aerosol stream.
 4. Method of manufacturing powder particlesaccording to claim 3, wherein both the first and the second aerosolstream comprise liquid charged particles.
 5. Method of manufacturingpowder particles according to claim 4, wherein the first aerosol streamis generated using a liquid which is miscible with the liquid used togenerate the second aerosol stream, and further comprises reacting theat least one particle in the first stream with the at least one particleof the second stream.
 6. Method of manufacturing powder particlesaccording to claim 4, wherein the first aerosol stream is generatedusing a liquid which is immiscible with the liquid used to generate thesecond aerosol stream, and further comprising reacting the at least oneparticle in the first stream with the at least one particle of thesecond stream.
 7. Method of manufacturing powder particles according toclaim 5, wherein the liquid of the first aerosol stream comprises afirst reagent and the liquid of the second aerosol stream comprises asecond reagent and further comprising chemically reacting the at leastone particle in the first stream with the at least one particle in thesecond stream.
 8. Method of manufacturing powder particles according toclaim 6, wherein the liquid of the first aerosol stream comprises afirst reagent and the liquid of the second aerosol stream comprises asecond reagent and further comprising chemically reacting the at leastone particle of the first stream with the at least one particle of thesecond stream.
 9. Method of manufacturing powder particles according toclaim 1, wherein at least the first aerosol stream comprises largerparticles and satellite particles, further comprising subjecting thefirst stream to a particle separation step providing a substantiallymonodisperse aerosol stream whereafter the substantially monodisperseaerosol stream is contacted with the second aerosol stream.
 10. Methodof manufacturing powder particles according to claim 9, wherein both thefirst and the second aerosol stream are subjected to the particleseparation step.
 11. Method of manufacturing powder particles accordingto claim 9, wherein the separation step comprises removing particlesfrom the stream using a grounded electrode.
 12. Method of manufacturingpowder particles according to claim 10, wherein the separation stepcomprises removing particles from the stream using a grounded electrode.13. Method of manufacturing powder particles according to claim 3,comprising evaporating the solvent from the first aerosol stream to forma first aerosol stream comprising solid, sticky charged particles. 14.Method of manufacturing a powder particle according to claim 1 whereinsubstantially neutral particles are separated from substantially chargedparticles.
 15. A powder comprising ceramic precursor powder particlesmanufactured according to claim
 1. 16. A powder comprisingpolymer-comprising particles manufactured according to claim
 1. 17. Apowder comprising a pharmaceutical compound-comprising powder particlemanufactured according to claim
 1. 18. A powder according to claim 17,wherein the pharmaceutical compound comprising powder particle is acoated powder particle.
 19. A powder according to claim 17, wherein thepowder comprises catalyst-comprising powder particles.
 20. A method ofmanufacturing a powder according to claim 15, wherein the ceramicprecursor powder is sintered.
 21. Pharmaceutical composition comprisinga powder comprising at least one pharmaceutically active compound,wherein the powder is a powder according to claim 17, together with apharmaceutically acceptable carrier or diluent.
 22. Pharmaceuticalcomposition comprising a powder comprising at least one pharmaceuticallyactive compound, wherein the powder is a powder according to claim 18,together with a pharmaceutically acceptable carrier or diluent.